In normal human hearing, acoustical energy in the form of sound waves is directed into the ear canal of a human by the outer ear. The sound waves impinge upon a tympanic membrane, i.e. the eardrum, located at the inner end of the outer ear canal. The pressure of the sound waves causes tympanic vibrations in the eardrum, thereby producing mechanical energy.
Three interconnected bones referred to as the ossicular chain transfer these tympanic vibrations of the eardrum across the middle ear and into the inner ear. The ossicular chain includes three major bones, the malleus, the incus and the stapes. The stapes terminates at a membrane referred to as the oval window, which serves as the outer boundary for the inner ear.
Mechanical vibrations conducted to the oval window generate fluidic motion within the inner ear. A spiral shaped portion of the inner ear, referred to as the cochlea, includes auditory receptor cells connected to the ends of auditory nerve fibers. Fluid vibrations within the inner ear actuate the receptor cells, thereby causing the nerve fibers to transmit signals to the brain which are perceived by the subject as sound.
Generally, hearing difficulties fall into one of two categories. Conductive hearing loss relates to the inability, or inefficiency, in mechanically conveying the vibrations caused by sound waves through the outer ear, the middle ear and the oval window to the fluid of the inner ear. Sensorineural hearing impairment relates to deterioration of the receptor cells and/or nerve fibers within the inner ear, so that fluidic vibrations within the inner ear are not sensed at all, or are sensed at a lower magnitude.
Over the years, various devices or aids have been developed to improve the hearing of hearing impaired individuals. One such device is generally referred to as an externally worn hearing aid. This device amplifies processed sound waves in the external ear canal. While it has been estimated that 20% of hearing-impaired individuals have purchased a hearing aid, it is also reported that less than one-half of these individuals wear their hearing aids regularly.
Externally worn hearing aids suffer from several inherent problems which result in distorted hearing and a poorly tolerated device. First, the amplifying of sound waves in the external ear canal while the external canal is obstructed with a hearing aid produces constructive and destructive acoustical wave interference. This interference results in resonance of some frequencies, cancellation of other frequencies and distortion of the remaining acoustical waves.
Second, because of the relative proximity of the hearing aid microphone and speaker, acoustical feedback is a constant problem, producing "whistling and screeching" of the hearing aid when amplification is turned up. The more amplification required, the worse this problem becomes. While some hearing aids employ a tight-fitting mold to reduce this feedback, such a mold is usually uncomfortable, and often ulcerates the skin of the ear canal or produces autophony, i.e. the hearing by a patient of his or her own voice in that ear. Moreover, radiation of acoustical output back into the microphone via the hearing aid case or the hearing aid internal components further limits the gain/output of an externally worn hearing aid.
Third, these hearing aids provide only limited amplification, due primarily to the limited power from a hearing aid battery. Commercially available in-the-ear and behind-the-ear hearing aids amplify sound by a magnitude of about 30-70 dB.
Fourth, distortion of hearing aids is high. Compared with radios, stereo sets and other electronic devices, the electronic distortion of hearing aids is enormous. Average commercially available hearing aids have a total harmonic distortion of 2-25%. Transient and intermodulation distortion produce further acoustical problems. The signal-to-noise ratio of commercially available hearing aids is vastly inferior to even inexpensive sound systems.
Because of this acoustical and electronic distortion, signal processing is usually required in the form of band pass filters, noise suppression circuits, etc. These electronic circuits further drain the power source and limit amplification.
Fifth, externally worn hearing aids cannot be safely worn by a significant number of individuals whose hearing is impaired by diseases which affect the external ear canal or middle ear, such as congenital aural atresia with absent ear canal, external otitis, chronic otitis media, mastoiditis, eardrum preformation, etc.
Sixth, externally worn hearing aids cannot be effectively worn when playing contact sports, perspiring excessively, swimming, showering, working in excessive noise and in many other conditions.
Finally, externally worn hearing aids often carry a social stigma, particularly in children. This social distinction can adversely affect a child's positive self esteem.
As a result of these problems, a number of semi-implantable hearing devices have been developed. These hearing devices actuate the inner ear either electromagnetically or by a piezoelectric bimorph lever. However, after nearly thirty years of attempts to develop a practical electromagnetically or piezoelectrically actuated hearing aid, to applicant's knowledge, none of these devices have yet been approved in the United States by the FDA. This lack of success is the result of problems inherent in each of these approaches, problems which have not yet been solved.
Electromagnetic actuation devices have been unsuccessful for several reasons. First, the strength of the magnetic field which actuates the ear is directly dependent on the amount of current flowing through the magnetic coil and the number of turns in the coil. Thus, high current and/or a coil with an extremely large number of turns is required. For a conventionally sized coil, this high current requirement rapidly drains battery power, exhausting a conventionally sized battery source within several hours.
Second, the amount of amplification produced in the core magnet is approximately inversely proportional to the square of the distance between the induction coil and the core magnet. Third, these electromagnetic actuation devices may be susceptible to stray magnetic fields. Finally, in clinical trials in the United States, optimum amplification of electromagnetic actuation devices has been in the range of only about 30-40 dB.
The inherent flaw with piezoelectric bimorph lever relates to size. More specifically, a lever of unrealistic length is necessary to attain adequate amplitude of sound vibrations to stimulate the middle ear ossicles. The middle ear is simply too small to accommodate the necessary piezoelectric lever length.
Presently, in Japan, surgeons are attempting to inertially anchor a piezoelectric bimorph lever in the mastoid. However, these procedures require major destructive otologic surgery, including radical mastoidectomy and closure of the ear canal. To the extent that the implanting of such devices requires destructive procedures, these devices are not likely to be approved in the United States by the Food and Drug Administration.
Perhaps more importantly, to the extent that implantable devices or procedures of this type do not result in improved hearing, the situation is irreversible, and the subject will most likely have lost any opportunity for hearing improvement by other implantable devices or surgical procedures.
It is an objective of this invention to overcome the present problems associated with commercially available, externally worn hearing aids via an auditory system which is sufficiently safe and reliable to achieve F.D.A. approval.
It is another objective of the invention to develop an implantable auditory system, and particularly an actuation device, with reduced electrical power requirements, better acoustical amplification, and which is small enough to eliminate the need for major and/or destructive surgical procedures.
It is still another object of the invention to develop an implantable auditory system with a high probability of success in overcoming a subject's conductive and/or sensorineural hearing deficiency, but which does not cause irreversible hearing loss in the subject if the system should wear out or prove to be unsuccessful.
The above-stated objectives are achieved by an implantable auditory system which comprises a micromachined microsensor and a micromachined microactuator which are very small, yet which provide up to 100 dB of amplification. Because of the small size, surgical implanting of these components within the middle ear of a subject requires no destructive and/or irreversible surgical procedures.
To the contrary, present surgical techniques, including laser surgery, may be used to implant these micromachined components. In fact, according to one embodiment of the invention, the micromachined actuator employed by this auditory system may be incorporated into the bottom of a piston-like prostheses which is extended through the stapes footplate in present stapedotomy techniques.
The auditory system of this invention is an integrated, fully implantable micro system which improves hearing in patients with conductive and/or sensorineural deafness. This auditory system utilizes silicon semiconductor microfabrication and micromachining techniques to produce integrated components which amplify hearing by electrostatically stimulating the fluid of the inner ear. Because of the size and configuration of the micromachined components, particularly the microactuator, which acts as a parallel plate capacitor, small voltage changes produce large electric fields which are used to vibrate the fluid of the inner ear.
According to the invention, the major components of this auditory system include a microsensor and a microactuator implanted in the middle ear of the subject and a signal processor, amplifier, and power source implanted subcutaneously in the cortical mastoid bone.
The microsensor is either a micromachined piezoresistive vibration sensor, a micromachined parallel plate capacitor, or a micromachined acoustical microphone designed and produced using microfabrication techniques and having a mass of less than about 30 grams. The microsensor senses acoustical pressure waves produced in the middle ear by mechanical vibrations of the eardrum or mechanical vibrations of one of the bones of the ossicular chain, and it converts the sensed waves or vibrations into electrical signals. The microsensor may be secured to one of the bones of the ossicular chain, preferably the incus. Alternatively, particularly for those individual subjects who suffer from congenital aural atresia, wherein the external canal is absent, the microsensor may be planted subcutaneously in the mastoid cortical bone behind the ear. If desired, the microsensor may be combined as an integral piece with the micromachined microactuator. As yet another alternative, the sensor may be inserted into the incudostapedial joint, i.e. the joint between the incus and the stapes, and used to sense the pressure therebetween.
After the microsensor converts sensed mechanical vibrations into electrical signals, the electric signals pass from the microsensor through the facial recess to a signal processor, which includes a signal conditioner, an amplifier, and a power source. The conditioned and amplified signal is then transmitted back to the microactuator located in the middle ear, which includes a flexible dielectric or semiconductive diaphragm on a semiconductor substrate that transduces the electrical signals back into mechanical vibrations to directly stimulate the perilymph fluid of the cochlea through a fenestration in the promontory or the stapes footplate. Alternatively, the diaphragm of the microactuator mounts to a piston which resides in contact with the incus or the stapes so that vibrations of the diaphragm and piston amplify the vibrations of the incus or the stapes, respectively, thereby indirectly stimulating the perilymph fluid. This latter alternative avoids the necessity of surgically entering the inner ear. This implanted auditory microsystem does not rely on amplification of sound waves in the external ear canal, and thus eliminates the substantial acoustical and electronic distortion created by present day externally worn hearing aids.
The microactuator is preferably a micromachined parallel plate capacitor with a major portion of a semiconductor crystal serving as one stationary plate and a flexible monolithic dielectric or semiconductive diaphragm spaced about 1-5 microns away from the major portion, with the spacing or void therebetween formed by etching. A metallized coating deposited on the exterior surface of the diaphragm may serve as the other "plate". Alternatively, if mounted in a fenestration in the promontory or stapes footplate and in contact with the perilymph, which is inherently electrically conductive, the perilymph may serve as the other "plate". The crystal may be doped. The doping of the crystal will dictate the etchants to be used in micromachining this capacitor.
Because of the extremely small thickness of the diaphragm and the void, voltage changes conveyed to the microactuator produce extremely high electric fields across the "plates" of the micromachined capacitor. The resultant electrostatic forces acting upon the plates cause the diaphragm to flex. By locating the microactuator in a position where the diaphragm movements can vibrate the fluid of the inner ear, voltage changes conveyed to the micromachined microactuator actuate the auditory receptor cells to cause the associated nerve fibers to signal the brain to perceive sound.
According to one approach, the flexible diaphragm is a dielectric which resides in direct contact with the perilymph fluid of the inner ear. According to this approach, the microactuator is housed within a screw which is threaded through a promontory fenestration formed via laser or other surgery techniques. The diaphragm is located at the end of the screw. The external surface of the screw serves as one electrode, and contact between the inserted end of the screw and the ionic perilymph fluid causes the perilymph fluid to act as one conductive plate which resides in direct contact with the flexible dielectric diaphragm.
An electrical lead extended through the screw conveys electrical voltage signals to the stationary plate of the microactuator, i.e., the major portion of the semiconductor crystal, which is spaced from the flexible diaphragm. When using this approach, it is important to electrically insulate the doped semiconductor material, which serves as the stationary plate of the microactuator, from the electrically conductive portions of the screw. This may be accomplished via a glass coating on the inside of the metal screw, or the use of a teflon screw coated on its external surface with an electrically conductive and biocompatible material such as gold.
This approach may also be used with a piston, rather from a screw, and by forming the fenestration in the stapes footplate, as in present stapedotomy techniques. In this present stapedotomy technique, passive sound transmission to the inner ear is achieved via mechanical vibration of a prostheses surgically extending through the fenestration in the footplate. One embodiment of the present invention modifies this prostheses by housing a micromachined capacitor in a piston extending into a fenestration in the footplate. Electronic actuation of the capacitor vibrates the diaphragm to amplify the vibrations of the piston. In this approach, if the electronics should happen to fail, the subject is no worse off because the mechanical piston, similar to presently used prostheses, is still in place.
Alternatively, the microactuator may be located in a micromachined semiconductor housing which is tapered to fit within a tapered fenestration in the promontory. In this approach, the flexible diaphragm may carry a conductive coating to serve as one of the plates, but preferably this perilymph is again used as the outer plate.
As another alternative, which is most applicable to a fenestration formed in the promontory, the diameter of the diaphragm may be increased and/or the diameter of the fenestration may be reduced by connecting the flexible diaphragm to a piston and locating the diaphragm outside of the inner ear and the piston inside the inner ear.
As yet another approach, the microactuator may be mounted to the incus or the stapes, and inertially grounded to the promontory if desired, so that movement of the flexible semiconductive diaphragm amplifies movement of the ossicular chain and thereby indirectly vibrates the perilymph fluid of the inner ear.
This inventive auditory system is made possible only because of the small geometries attainable with the revolutionary "micromachining" processes which may be applied to single crystalline &lt;100&gt; oriented semiconductor silicon. With a microactuator formed by micromachining and having dimensions and geometries that are this small, i.e. in the micrometer range, a relatively small voltage of a few volts can produce an enormous electrical field intensity which is entirely contained within the microactuator. As a result, strong electrostatic forces are generated which achieve a high degree of acoustical actuation in the very small spaces available in the middle ear and inner ear of a subject. Moreover, because this acoustical actuation is achieved via electrostatic forces produced by a micromachined capacitor, which uses only minimum electrical current, battery life for this auditory system is much longer than prior implantable systems. Preliminary studies indicate that an implanted five or six volt battery used to electrically drive the components of this auditory system can last up to four to five years.
Moreover, unlike implanted devices which rely upon electromagnetic actuation, stray magnetic fields can be expected to have virtually no effect on this auditory system. Likewise, stray electrical fields will have absolutely no effect on this system because the intensity of such stray electric fields will be several orders of magnitude lower than the intensity of the electric field produced in the electric fields generated in the microactuator used in this auditory system.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawings.